As disclosed in Japanese Patent Office Publication (JPOP) 09-186836, an image duplication device such as a digital photocopier utilizes digital data. The digital photocopier scans an input document and generates analog image data. The digital copier converts the analog scanner output data into output digital data format and stores it in a frame memory. Then, the digital copier outputs the digital image from the frame memory to a printer engine in order to generate a reproduction image. Alternatively, the digital photocopier stores the above digital data in a secondary memory unit such as a floppy disk or a hard disk, and at a later output time, the digital photocopier outputs the stored digital image data from the secondary memory storage unit to a printer engine. Similarly, the digital photocopier processes the stored digital image data into a print image signal in a frame memory while accessing font information in response to a print command from a terminal device such as a personal computer via LAN, serial interface and or parallel interface. The digital photocopier ultimately sends the processed image data to the printer engine.
In duplicating an image scanned from an input document, a digital photocopier scans a document that has printed information on both front and back of the page. It is possible to see through some dark image from the other side of a document since some image-carrying medium such as paper is semi-transparent. When a digital photocopier scans an image from one side of a double-sided document, a see-through image from the other side is sometimes undesirably scanned together. In order to avoid the see-through image, one exemplary prior art reference such as JPOP06-14185 discloses an image processing device determines a see-through image based upon an image intensity level histogram. The prior art image processing further utilizes the intensity value of the scanned see-through image portion as a color background to correct the see-through image area. Another exemplary prior art reference such as JPOP9-116753 discloses an image processing device that reads a predetermined see-through image on a certain image-carrying medium and cancels any see-through image from input image data.
Despite the above described prior art techniques, the see-through image problem remains to be improved. For example, when a see-through image is considered as a color background, and the intensity level is corrected as disclosed in JPOP06-14185, the gradation is sacrificed. A corrected output image cannot reproduce any image portion having an intensity level that is lower than that of the see-through image portion and lacks the accuracy. Furthermore, since the correction method is a discontinuous intensity correction, the output image has non-smooth appearance. Similarly, the cancellation method as disclosed in JPOP9-116753 also fails to produce a desirable image since the dynamic range of the image is limited. The current invention is directed to solve these and other problems related to the see-through images during image duplication.